She said the nurse had a student in her office who felt bad and had an irregular heartbeat after having a high-energy drink.

"The nurse asked me, 'How much caffeine is in those drinks?' "

The nurse was asking what more people - lured by Starbucks on every corner and Red Bull in every cooler - are wondering: How much caffeine is safe?

"It can vary for each person," says Dr. David Long, a family practice physician at University Medical Center. "But, roughly speaking, about 250 mg is safe. That's about two 5-ounce cups of coffee or five colas."

Any more than that, he said, and some people increase their risk for irregular heartbeat and rapid heartbeat.

Caffeine, the most widely used drug in the world, according to Bennett Weinberg's book "The World of Caffeine," is a stimulant in a group called xanthines, which occur in cocoa beans and even some leafy plants. As caffeine enters the bloodstream, it stimulates every cell in the body, which makes us feel more alert and awake, Weinberg writes.

Long said that some people, such as high school students with arrhythmia, may have a greater sensitivity to caffeine, and anyone with a heart ailment or high blood pressure should avoid caffeine or talk to a physician. For most of us, too much caffeine may cause us to be jittery, have an upset stomach, headaches or trouble sleeping, he said.

"Caffeine is considered a doping agent by the schools," Cook said. "Athletes found with too much caffeine in their system may be disqualified from competition."

He added that the limit is 1,200 mg, or about eight high-energy drinks.

When asked about long-term effects of caffeine, both doctors said the drug can aggravate, but does not cause, high blood pressure, heart problems and fibrocystic breast disease. Another long-term effect can be recurring sleep disturbance, which hurts overall health, Cook said.

Fillipp recommends reading product labels and keeping track of how much caffeine you consume. She also said that an herb called guarana, a caffeine-like stimulant, is now being added to products.

"So when you read labels, watch for guarana, too," she said.

"It's like the old saying," Fillipp said. "All things in moderation."

Caffeine content

12-oz. energy drink: 70-150 mg.

5 oz. coffee: 115 mg.

12 oz. iced tea: 70 mg.

12 oz. Mountain Dew: 55 mg.

12 oz. cola: 40 mg.

1 cold tablet: 30 mg.

12 oz. 7-Up/Sprite: 0 mg.

Source: U.S. Food and Drug Administration and National Soft Drink Association